Semiconductor devices, e.g. appropriate, integrated (analog or digital) computing circuits, semiconductor memory devices such as functional memory devices (PLAs, PALs, etc.) and table memory devices (e.g. ROMs or RAMs, in particular SRAMs and DRAMs), etc. are subject to comprehensive tests e.g. in the finished and/or semifinished state.
In the scope of such kind of test, for instance with particular DRAMs (Dynamic Random Access Memories or dynamic read-write memories, respectively), in particular with DDR DRAMs (Double Data Rate DRAMs), the output impedance of the driver devices provided in the chip or semiconductor device, respectively, may be adapted to the impedance of the signal lines.
Every driver device may e.g. comprise a pull-up and a pull-down circuit being connected in series.
The pull-up circuit is e.g. connected to the supply voltage, and the pull-down circuit is connected to the ground.
For outputting a “logic One” (during the regular operation of the semiconductor device), the pull-up circuit may be switched on, i.e. be put in a conductive state, and the pull-down circuit may be switched off, i.e. be put in a locked state—then, a “logically high” output signal will be output at an output pad connected between the pull-up and the pull-down circuits.
Correspondingly, for outputting a “logic 0”—during the regular operation of the semiconductor device—, the pull-up circuit is switched off, i.e. put in a locked state, and the pull-down circuit is switched on, i.e. put in a conductive state, so that, correspondingly, a “logically low” output signal is output at the output pad.
The pull-up and the pull-down circuits may e.g. each comprise a plurality of transistors connected in parallel (e.g. the pull-up circuit a plurality of p-channel MOSFETs and the pull-down circuit a plurality of n-channel MOSFETs).
The output impedance of the driver devices provided in the chip or in the semiconductor device, respectively, may be adapted during the testing operation of the semiconductor device (i.e. in the scope of an appropriate testing method) to the impedance of the signal lines for instance by the fact that, by means of a so-called laser fuse method from chip to chip (e.g. from driver device to driver device), a different number of transistors in the respective driver devices is put to the “enabled” state (i.e. is later on, during the regular operation of the semiconductor device, used for driving signals), or is left in a “non-enabled” state (i.e. is later on, during the regular operation of the semiconductor device, not used for driving signals).
In order to determine how many (or which) transistors are to be put in an “enabled” state by means of the laser fuse method, and how many (or which) transistors are to be left in a “non-enabled” state (so-called “driver setting”), the pull-up circuit may first of all be switched on and the pull-down circuit may be switched off (then, a “logically high” output signal level is driven by the driver device).
Subsequently, a reference voltage is applied to the above-mentioned output pad—e.g. by means of a corresponding signal needle—, and the current flowing from the supply voltage via the pull-up circuit to the output pad is then measured for all possible driver settings.
From the current measured, the respectively applicable output impedance of the pull-up circuit may be determined for the respective driver setting in a manner known per se (and thus that driver setting may be selected at which the output impedance of the pull-up circuit corresponds—as precisely as possible—to the desired pull-up circuit output impedance).
Subsequently (correspondingly vice versa), the pull-up circuit may be switched off and the pull-down circuit may be switched on (so that a “logically low” output signal level is then driven by the driver device), and—again for all possible driver settings—the current then flowing from the output pad via the pull-down circuit to the ground may be measured.
From the current measured, the respectively applicable output impedance of the pull-down circuit may then be determined for the respective driver setting in a manner known per se (and thus that driver setting may be selected at which the output impedance of the pull-down circuit corresponds—as precisely as possible—to the desired pull-down circuit output impedance (wherein this should be as identical as possible to the above-mentioned pull-up circuit impedance)).
In the case of this trimming method, however, the problem arises that—depending on the degree of dirtying of the signal needle used for measurement, via which, as explained above, a (reference) voltage is applied to the output pad—the signal needle contact resistance may vary strongly.
The respective signal needle contact resistance influences—in a way difficult to predict—the current flowing through the output pad (and thus—in a way difficult to predict—the respective value determined for the output impedance).
Therefore, with the above-described method, the output impedance can be set with relatively small accuracy only.